Aliens in the community: Consequences of plant invasions on compartmentalization and species' roles in plant pollinator networks

Compartmentalization – the organization of ecological networks into subgroups of species that are not connected by interaction links with other groups (compartments) or have a higher probability of interacting with one another than with other species (modules) – has been identified as a key property for the stability, functioning and evolution of
multitrophic communities. Invasions of often highly generalized alien species may lead to the fusion of compartments or modules and alter the functional architecture of networks through shifts in the distribution of distinct topological roles a species can play in the network. We tested these hypotheses for alien plant invasions of plant–pollinator
networks using a dataset of 44 paired networks from seven published studies, each pair consisting of an invaded and a control network lacking alien plant invaders. The number of compartments was indeed lower in invaded compared to networks without alien plants, but not the number of modules detected by simulated annealing. The effect of invasion on modularity (estimating between-module differentiation) was contingent on the study system. However, module size, i.e. the mean number of species forming a module, increased following invasion, also after accounting
for variation in network size. Moreover, plant invasions altered the composition of species’ topological roles; in particular, the average number of species acting as module hubs, i.e. species highly linked within but not among modules, almost doubled following invasion. We discuss the implications of our findings for the conservation and restoration of plant–pollinator communities in the face of biological invasions